The present invention relates to a metallic gasket and, more particularly, to a metallic gasket for use in an indirect engine in which hot plugs are fitted in the lower portion of a cylinder head.
An indirect engine, an indirect diesel engine in particular, comprises main combustion chambers (main chambers) and subsidiary combustion chambers (subsidiary chambers) each of which communicates with each of the main combustion chambers through a throttle. Fuel gas which is subjected to primary combustion and incomplete mixture in the subsidiary combustion chamber are made to pass through the throttle, are jetted into the main combustion chamber, and are then well mixed with air, which is present in the main combustion chamber, by jet energy, which occurs when they pass through the throttle, so as to be subjected to complete combustion. Since a structure in which hot plugs are fitted in a cylinder head is adopted in this indirect diesel engine, a step is formed between the bottom face of each of the hot plugs and the bottom face of the cylinder head. In cases where the bottom face of each hot plug is convex, stress concentrates on a bead plate (bead plates) and the others, which situation, in combination with thermal stress resulting from a high-temperature portion which is opposite to each hot plug, easily causes cracks in the periphery of each combustion chamber, which periphery includes the bead plate(s) and the others, and therefore there has been a problem of securing sealability.
For the purpose of coping with such problems, various types of gaskets, each of which is composed of four plates, namely, two bead plates and two intermediate plates held between those two bead plates, have been proposed as layered metallic gaskets for use in the indirect engines in particular.
For example, a metallic layered head gasket is disclosed in Japanese Utility Model Laid-Open Publication No. 6-24268 (1994) according to which, two intermediate plates, namely, an upper adjusting plate and a lower adjusting plate, are laminated between a first bead plate and a second bead plate, and the upper adjusting plate is folded over at the peripheral edge of each combustion chamber hole and at the peripheral edge of each cut-out hole, thereby forming overlap-folded edge parts (closely-folded parts). Moreover, a gasket in which a metallic shim plate or a soft member is sandwiched for reinforcement of each portion which is opposite to the hot plug and the others is disclosed in Japanese Utility Model No. 2575037, Japanese Patent Laid-Open Publication No. 8-42697 (1996), Japanese Patent Laid-Open Publication No. 8-42698 (1996) or the others.
Recently, in tandem with movement toward weight reduction which can be achieved by what is called conversion to all aluminum and higher combustion pressure for the engine, from the viewpoint of cost, a gasket which is composed of three plates is demanded for use in this type of indirect diesel engine. One example of a cylinder head gasket composed of three plates is disclosed in Japanese Utility Model Laid-Open Publication No. 2-83356 (1990) according to which, grommets are used in conjunction with wire rings. Another example is disclosed in Japanese Patent Laid-Open Publication No. 6-337069 (1994) according to which, a shim ring is formed about the periphery of each bore hole to constitute a stopper part, and a bead plate having annular beads is formed with presser beads which are respectively formed opposite the hot plugs.
Moreover, as prior arts that is relevant to the present invention, Japanese Patent Laid-Open Publication No. 9-196178 (1997) and Japanese Patent Laid-Open Publication No. 10-9392 (1998), which are proposed by the present applicant(s), each disclose a layered gasket composed of three plates. According to each of these prior arts, a thick intermediate plate provided with beads or beaded stoppers is held by a thin bead plate.
The foregoing conventional gaskets each of which is composed of four plates, however, reveal the following problem. Since one of the intermediate plates is formed with the closely-folded parts which are being folded at an angle of 180xc2x0 and serve as stopper parts, usable material is usually limited to, for example, a low elastic metal plate such as SUS304 or the like, from the viewpoint of prevention of cracks which occur during a bending operation carried out for formation of these folded parts. For this reason, although a stopper function can be imparted to this type of intermediate plate, it is difficult to impart a bead plate function thereto for sealing.
Moreover, according to the foregoing conventional structures, either the thick intermediate plate, or the thick adjusting plate, or both in a laminated condition are interposed between the two bead plates, so that sealing surface pressure of each of the bead plates does not reach the opposite side of the thick plate or the laminated plates at all. In other words, owing to these thick plates or the laminated plates, a sealing function is divided, and a sealing effect problematically declines. Furthermore, in the case of the gasket having the metallic shim plates each of which is used for reinforcement of the portion which is opposite to the hot plug and the others, strength of each shim plate becomes excessively high, and variation of the step of the bottom face of each hot plug is poorly absorbed. In the case of the gasket having the soft members, the soft members are problematically low in strength. Either of these cases has a problem that the number of materials as well as the number of man-hours to perform installation and to check whether there is no slipups in the installation increase.
Similarly to the gasket having the metallic shim plates and the gasket having the soft members, the foregoing gasket which is composed of three plates and has the wire rings and the others has a problem that the number of materials and the number of man-hours increase. Moreover, in the case of the gasket having the hot plug presser beads which are formed on the bead plate, it is difficult to secure strength of the presser beads, and even when their strength is increased, stress adversely concentrates on the presser beads per se and the bead plate which become cracked or deformed as a result.
In the case of those gaskets which are disclosed in Japanese Patent Laid-Open Publication No. 9-196178 (1997) and Japanese Patent Laid-Open Publication No. 10-9392 (1998) as being relevant to the present invention, each bead or each beaded stopper is simply formed about the periphery of the bore hole in the annular form, with no consideration given to reinforcement of each portion which is opposite to the hot plug of the indirect engine.
The present invention addresses the problems discussed above and aims to provide a metallic gasket which is suitable for use in an indirect engine, the metallic gasket realizing both sealing performance and cost reduction which can be achieved by the decreased number of constituting plates or the decreased number of materials.
The present invention is based on a finding that with the use of either a low elastic spring material or a material having good cold workability, heat resistance and high strength, even in cases where no stopper is provided, bore beads formed on such a material are hardly susceptible to high stress amplitude and high stress concentration which cause bead cracks and setting, and with the use of such a material as a second bead plate on which folded parts and the bore beads are formed, a desired sealing function can be obtained without the need to increase the number of plates to be laminated.
A metallic gasket according to the present invention is used in an indirect engine in which hot plugs are fitted in the lower portion of a cylinder head and comprises:
(a) a first bead plate being made of elastic metal and comprising hot plug beads which are respectively formed opposite the hot plugs; and
(b) a second bead plate being made of elastic metal and comprising folded parts each of which is formed about the periphery of a bore hole, and annular bore beads each of which is formed about the periphery of an area including the bore hole and a portion which is opposite to the hot plug.
According to the present invention, the first bead plate is formed with the hot plug beads, and therefore each hot plug bead supports the hot plug, and load is prevented from concentrating locally on each primary gas sealing-out portion which is also prevented from becoming deformed and cracked. Moreover, each hot plug is prevented from falling. Since the first bead plate is not a complete rigid body, it functions as a margin for absorbing stress, which results from a step of the bottom face of each hot plug and variation of the step, and functions as a thickness adjusting plate. On the other hand, the second bead plate is appropriately formed with the folded parts, and therefore the increased thickness can be provided for the periphery of each bore hole, and in cases where the metallic gasket is composed of three plates, each folded part serves as a stopper with respect to each bore bead formed on a third bead plate. Moreover, each bore bead formed on the second bead plate is an annular bead which is formed about the periphery of an area including the bore hole and the portion which is opposite to the hot plug, so that a clearance caused by the step which is formed between the bottom face of each hot plug and the bottom face of the cylinder head is not made on a seal line of each annular bore bead. In other words, the periphery of each bore hole can be sealed without fail.
In the present invention, it is preferable that the first bead plate end portions which respectively face the bore holes are respectively held by the folded parts formed on the second bead plate. Thus, with the thickness of the thin second bead plate or with the thickness which is smaller than the thickness of the second bead plate, the stopper amount which is a little smaller than the height of the hot plug bead can be secured, and load occurring for the time during which the gasket is clamped between the cylinder head and a cylinder block can be appropriately concentrated on each folded part, so that appropriate high surface pressure occurs on each portion which is close to the bore hole. In addition, the first bead plate end portions which respectively face the bore holes are respectively protected from the high-temperature hot plugs by being respectively held by the folded parts, so that cracks resulting from thermal stress are prevented from occurring in each end portion which faces the bore hole.
It is preferable that corner portions of each of the first bead plate end portions held by the second bead plate, which corner portions extend along the periphery of the bore hole, are chamfered. Thus, cracks resulting from high temperature stress can be prevented from occurring in folded round parts-of the second bead plate. It should be noted that the chamfering can be carried out separately from or together with the folding operation.
In the present invention, the first bead plate and the second bead plate can also be laminated in such a manner that the first bead plate end portions which respectively face the bore holes are not respectively held by the folded parts formed on the second bead plate, while the inner side of each of the folded parts is provided with a clearance. In this case, curved parts are preferably prevented from cracking, and the clearances are preferably maintained even after the thus-configured gasket is clamped between the cylinder head and the cylinder block.
With consideration given to both prevention of cracks in the folded round parts and maintenance of bead elasticity, it is better that the second bead plate be made from a spring material having cold workability.
In the present invention, it is preferable that a third bead plate which is made of elastic metal and comprises bore beads is laminated on the first bead plate. Compared with the gasket composed of two plates, this decreases the number of leakage paths between layers, so that improved sealing reliability can be obtained. Moreover, the thickness of the first bead plate is appropriately set, so that if a thick intermediate plate and/or a thick adjusting plate is not interposed between the second bead plate and the third bead plate, sealing surface pressure of each of the second and third bead plates reaches the opposite side of the first bead plate across the first bead plate. In other words, owing to the first bead plate interposed between the second and third bead plates, sealing surface pressure of each of the second and third bead plates is not divided, so that a good sealing effect can be achieved. Furthermore, with the adoption of a structure in which the first bead plate end portions which respectively face the bore holes are respectively held by the folded parts of the second bead plate, the gasket composed of such three plates can have excellent heat resistance and excellent crack resistance about the respective peripheries of the bore holes. This effect is brought to the fore particularly when the second bead plate formed with the folded parts is made from austenitic high manganese stainless steel.
Here, it is better for the first bead plate to have an appropriate thickness which is basically greater than those of the second bead plate and the third bead plate. Thus, the first bead plate becomes a constituting plate having strength and cold workability and therefore can minimize cracks and deformation. It should be noted that the third bead plate is preferably made from a spring material.
Each hot plug bead formed on the first bead plate comes in various shapes such as convex beads, half beads, stepped beads and the others. Specifically, each hot plug bead is shaped into linear beads which are arranged in parallel relation, elliptical beads which are arranged in parallel relation, double half beads or plural beads in the form of small projections. The shape, the height and the overall structure of each hot plug bead can be selected in an appropriate manner in accordance with the step formed between the sealing contact face and the bottom face of the hot plug, required stress dispersion, required surface pressure and the others. It should be noted that with consideration given to the simple shape, easy working, effects and the others, each hot plug bead is most preferably shaped into a convex bead which is crescent-shaped in a plan view.